1. Technical Field
The present invention relates to a printhead and a printing apparatus including a printhead, and more particularly, to an inkjet printing apparatus in which ink is supplied from a main tank holding a large amount of ink to a printhead through a sub-tank.
2. Background Art
Recently, inkjet printing apparatuses are widely used as printers of personal computers or copying machines. Since the inkjet printing apparatus is inexpensive and capable of full-color printing, the apparatus' demand is increasing. Also, the technique of inkjet printing is applied in photographic image printing, in which multi-tone image printing is necessary.
Such a multi-tone image printer can express a much larger number of tones than the actually mounted number of ink types by using in combination plural types of ink having different densities and superimposing the plural types of ink a couple of times. Recently, inkjet printers are also used in plotters which print photographic images on a large-size sheet, such as A1 (594 mm×841 mm) and A0 (841 mm×1189 mm).
However, the above-described plotter and photographic image printer consume a large amount of ink. If an ink tank is mounted on the carriage together with a printhead, the ink tank must be exchanged frequently, and it is inconvenient.
Such an inkjet printer consuming a large amount of ink generally uses an ink supply system shown in FIG. 15. FIG. 15 is a schematic diagram of an ink supply system in a conventional inkjet printing apparatus.
As shown in FIG. 15, a printhead H301 is mounted on a carriage H302 which is movable with respect to the apparatus main body. A main tank M303 is fixed to the apparatus main body. The main tank M303 is arranged to be exchangeable when the remaining ink in the main tank M303 is running low.
The printhead H301 is connected with the main tank M303 by an ink flow passage M304, which is consisted of tubes and joints or the like. The carriage H302 moves reciprocally when printing is executed. The motion of the carriage H302 is not disturbed because a flexible tube (e.g., silicon tube or polyethylene tube) is used in at least part of the ink flow passage M304.
The main tank M303 has an air communication hole M305. The inner part of the main tank M303 is in communication with the atmosphere. When the printhead H301 discharges ink, ink is refilled from the main tank M303 to the printhead H301 through the ink flow passage M304.
Inside the printhead H301, there is a sub-tank H310 which directly receives ink from the ink flow passage M304. Below the sub-tank H310 is an individual liquid chamber H309 provided through a filter H307. The individual liquid chamber H309 serves as an ink reserving area for introducing the ink to a printing element substrate H308.
The pressure inside the printhead H301 must be maintained in a negative pressure state so that the ink does not leak from the discharge orifice H306. The pressure inside the printhead H301 is determined by the ink level of the main tank M303. It is preferable that the ink level of the main tank M303 be set at the position lower by 20 mm to 100 mm than the position of the discharge orifice H306 of the printhead H301.
This method can realize ink supply with an extremely simple configuration. However, since the flexible tube used in the ink flow passage M304 is made of rubber or resin, the tube has slight gas permeability.
Because the inner portion of the tube has negative pressure similarly to the inner portion of the printhead H301, air from the atmosphere infiltrates the inner portion of the tube little by little through the tube wall, and bubbles are generated. If the bubbles flow into the sub-tank H310 in the printhead H301, maintaining the negative pressure inside the printhead becomes difficult. Furthermore, ink supply to the individual liquid chamber H309 becomes insufficient and regular ink droplets cannot be discharged. This causes printing defects.
Even if it were possible to prevent air from infiltrating the inner portion of the tube little by little through the tube wall, there is a possibility that air dissolved in the ink may grow into bubbles inside the tube or sub-tank H310. Moreover, there is a possibility that bubbles get inside the sub-tank H310 from the atmosphere.
In view of the above situation, an ink supply system shown in FIG. 16 is proposed as a method for preventing bubbles from getting in the printhead H301 even if bubbles are produced, and furthermore for removing bubbles in the sub-tank by circulating the bubbles.
FIG. 16 is a schematic diagram of another ink supply system in a conventional inkjet printing apparatus. FIG. 16 shows a printhead H301, and a sub-tank H310 which is provided inside the printhead H301 for reserving ink to be supplied to a printing element substrate H308.
The printhead H301 and the sub-tank 310 are mounted on a carriage H302 which is movable with respect to the apparatus main body. A main tank M303 is fixed to the apparatus main body. The main tank M303 is exchangeable when the remaining ink in the main tank M303 is running low. The printhead H301 is connected with the main tank M303 by two ink flow passages: first and second ink flow passages M3041 and M3042, each consisted of tubes and joints.
The first ink flow passage M3041 transfers the ink contained in the main tank M303 to the sub-tank H310. The second ink flow passage M3042 transfers back the ink in the sub-tank H310 to the main tank M303. Provided in midstream of the second ink flow passage M3042 is a pump M311 that generates ink flow by a piston or by rotating plural rollers. The ink in the sub-tank H310 is sent to the main tank M303 by the pump M311.
The main tank M303 has an air communication hole M305. The inner part of the main tank M303 is in communication with the atmosphere. However, since the sub-tank H310 has an airtight structure, the inner part of the sub-tank H310 is not in communication with the atmosphere. Therefore, when the pump 311 is driven, the ink in the sub-tank H310 is sent to the main tank M303 through the second ink flow passage M3042, while the ink in the main tank M303 is sucked by the sub-tank H310 through the first ink flow passage M3041. In this manner, ink circulation is executed between the sub-tank H310 and the main tank M303.
The pressure inside the sub-tank H310 must be maintained in a negative pressure state so that the ink does not leak from the printhead H301. The pressure inside the sub-tank H310 is determined by the ink level of the main tank M303. It is preferable that the ink level of the main tank M303 be set at the position lower by 20 mm to 100 mm than the position of the printhead H301 (discharge orifice surface).
According to the above-described configuration, bubbles are generated in the first ink flow passage M3041 similarly to the conventional art shown in FIG. 15. However, the bubbles enter the sub-tank H310, flow through the second ink flow passage M3042, and end up being discharged to the main tank M303. Therefore, according to the above-described proposed configuration, the bubbles generated in midstream of the ink flow passage do not enter the sub-tank H310 in the printhead H301.
There is another conventional technique, which is disclosed in Japanese Patent Laid-Open No. 8-244250, for removing bubbles by circulating ink between a sub-tank and a main tank.
However, in the aforementioned plotters and medical image printers, there is a trend toward increasing types of ink for expressing more complicated tones.
For instance, in photographic color plotters, using more than six colors of ink, which include low-density cyan and magenta in addition to regular cyan, magenta, yellow, and black, has been proposed. Also, in medical image printers, at least six densities of black ink are necessary in order to print an image, for example, an X-ray image, where more than 1000 tones are necessary.
In a case where six types of ink are used, six ink supply systems must be provided, and twelve (6×2=12) ink flow passages are necessary between the sub-tank and the main tank.
As mentioned above, air from outside infiltrates the inner portion of the tube. It has also been confirmed that moisture and solvent in the ink evaporate outside the tube through the tube wall. Therefore, the more the number of ink flow passages between the sub-tank H310 and the main tank M303, the more the moisture and solvent evaporate, and as a result, the ink density changes. In a multi-tone image, particularly in an image having more than 1000 tones, accurate tone expression cannot be realized if the density of each ink changes.
Although the configuration for circulation shown in FIG. 16 can remove bubbles that get mixed inside the sub-tank H310 by a circulating operation, the circulating operation cannot be performed in the individual liquid chamber H309 provided below the sub-tank H310. To remove bubbles in the individual liquid chamber H309 which have been generated by printing or the like, a compression mechanism M306 is operated, and the individual liquid chamber bubbles H3091 are removed by running ink from the discharge orifice H306. This generates waste ink.
Furthermore, in the method of removing individual liquid chamber bubbles H3091 by running ink using the compression mechanism M306, there is a possibility that the sub-tank bubbles H3101 may move to the individual liquid chamber H309. When the bubbles pass through the filter H307, they become creamy fine bubbles. These fine bubbles tend to remain inside the individual liquid chamber H309 even if the compression mechanism M306 is operated. In fact, a large number of bubbles still may remain even after the recovery operation. In view of this, Japanese Patent Laid-Open No. 2000-301737 proposes a configuration for removing bubbles by providing a float valve inside the sub-tank. Moreover, Japanese Patent Laid-Open No. 11-320901 and No. 2006-095868 propose a configuration for removing air inside the sub-tank by providing a communication hole having a check valve in the ink chamber and releasing the check valve by a mechanism outside the printhead.